Patients treated by BiPAP had significantly more adverse events. Two BiPAP treated patients died versus zero in the high dose ISDN group. Sixteen BiPAP treated patients (80%) required intubation and mechanical ventilation compared to four (20%) in the high dose ISDN group (p = 0.0004). Myocardial infarction (MI) occurred in 11 (55%) and 2 (10%) patients, respectively (p = 0.006). The combined primary end point (death, mechanical ventilation or MI) was observed in 17 (85%) versus 5 (25%) patients, respectively (p = 0.0003). After 1 h of treatment, oxygen saturation increased to 96 ± 4% in the high dose ISDN group as compared to 89 ± 7% in the BiPAP group (p = 0.017). Due to the significant deterioration observed in patients enrolled in the BiPAP arm, the study was prematurely terminated by the safety committee.

CONCLUSIONS

High dose ISDN is safer and better than BiPAP ventilation combined with conventional therapy in patients with severe pulmonary edema.

Continuous positive airway ventilation (CPAP) and bilevel positive airway ventilation (BiPAP) are being increasingly used in the treatment of acute respiratory failure and pulmonary edema (1). However, to date and to our knowledge, no large randomized trials have compared this treatment with other treatment modalities. We have recently demonstrated (2) that the use of intravenous (IV) high dose isosorbide-dinitrate (ISDN) in the treatment of severe pulmonary edema improves control of respiratory failure, and reduces the need for mechanical ventilation and the rate of myocardial infarction (MI). Since the two treatment strategies are commonly used in our institution as well as other emergency departments in Israel, we have undertaken a study in which Bi-PAP ventilation combined with conventional treatment was compared to high dose ISDN in patients with severe pulmonary edema.

Ethical considerations dictated the different ISDN dose in the BiPAP and control group. Patients in the BiPAP arm were treated by BiPAP ventilation and continuous IV ISDN. High dose IV ISDN was not coadministered with BiPAP ventilation due to concerns about a possible hypotensive effect of such treatment combination. The use of standard-dose continuous IV ISDN as a single treatment method was considered unethical by the hospital review board due to the results of our previous study (2). Therefore, we compared in a prospective randomized study the efficacy and safety of BiPAP ventilation versus IV high dose nitrate therapy in patients with severe pulmonary edema.

Methods

Between January and June 1999, 40 consecutive patients with severe pulmonary edema were recruited for the present study. The study protocol was approved by the hospital and national ethical review board. Severe pulmonary edema was defined as symptoms and signs of pulmonary edema accompanied by oxygen saturation of <90% measured by pulse oximetry upon hospital admission, prior to oxygen administration.

Exclusion criteria were as follows:

1. previous treatment with nitrates above 40 mg/d, or mono-nitrates or long-acting tri-nitrates administered more than twice daily or short acting tri-nitrates administered more than three times a day;

2. previous treatment with furosemide >80 mg/d;

3. hypotension (blood pressure <110/70 mm Hg);

4. previous adverse effect of nitrates;

5. ST elevations consistent with acute MI on baseline ECG; and

6. absence of pulmonary edema on chest radiograph obtained on arrival to the emergency department.

On hospital admission, each patient was placed in sitting position and oxygen was administered by facemask with a rebreathing bag at a rate of 10 liter/min. An IV line was inserted and an IV bolus of morphine 3 mg and furosemide 80 mg was administered. Informed consent was obtained. Heart and respiratory rates, blood pressure and oximetric O2 saturation were obtained at baseline and every 3 min during treatment. Randomization was performed by assigning consecutive patients to one or other of the treatment groups according to their numerical order on a list that had been predetermined by lot.

Patients were randomized to receive one of two treatments:

1. BiPAP and conventional treatment (n = 20): the BiPAP was administered using a BiPAP ventilatory assist system (Respironics), a pressure-limited device that cycles between adjustable (up to 20 cm H2O) inspiratory and expiratory pressures using patient flow-triggered (S) mode. The inspiratory positive airway pressure (IPAP) was set at 8 cm H2O initially, and the expiratory positive airway pressure (EPAP) was set at 3 cm H2O. Supplemental oxygen was blended in via a mask port at a rate of 10 liter/min. Patients were encouraged to coordinate their breathing with the ventilator. During the trial, IPAP was increased by 1 cm H2O every 3 to 4 min as tolerated and up to 12 cm H2O. Subsequent EPAP was increased by 1 cm H2O every 3 to 4 min up to 5 cm H2O. Patients were encouraged to use BiPAP for as long as tolerated, aiming for at least 50 min. Masks were tightened just enough to control air leakage. Concomitantly IV ISDN continuous drip was started with 10 μmol/min and increased every 5 to 10 min by 10 μmol/min.

The randomization and treatment of pulmonary edema were administered by mobile intensive care unit teams in the patient’s home or during delivery to the emergency department. During the study period, no other drug beside protocol study drugs was administered.

IPAP and EPAP as well as ISDN dose up-titration in group 1 and repeated ISDN boluses in group 2 were continued in both groups until the oxygen saturation increased above 96% or systolic blood pressure dropped below 110 mm Hg systolic or 30% below baseline levels. Patients with oxygen saturation below 80% despite therapy or increasing dyspnea accompanied by altered neurologic status were intubated and mechanically ventilated. Additional morphine was administered only prior to intubation.

Primary end points were as follows: adverse events including death, need for mechanical ventilation or MI within 24 h of hospital admission. Myocardial infarction was defined as an increase of CK to more than twice the upper limit of normal of our institution accompanied by an increase in CK-MB to >6%.

Secondary end points were as follows: speed of recovery from pulmonary edema as reflected by a decrease in pulse and respiratory rate and increase in oxygen saturation.

Statistical analysis

Comparison between the two treatment groups regarding baseline parameters, treatment and primary end points was performed using the two-tailed Student t test to compare continuous variables and the Fisher exact test to compare the distribution of categorical variables. Differences in O2 saturation, respiratory and pulse rate changes over time were calculated by one-way analysis of variance (ANOVA) with repeated measures. Results are expressed as mean ± SD. p values <0.05 were considered statistically significant.

Results

Patient recruitment is presented in Figure 1. Baseline characteristics of patients in both groups are presented in Table 1. Treatment with IV ISDN, furosemide and morphine is presented in Table 2. The decrease in mean arterial blood pressure was similar in both groups (Table 3).

Primary end point (clinical outcome)

Patients treated by BiPAP ventilation had significantly more adverse events. Two patients (10%) died in the BiPAP arm as compared to 0 (0%) in the high dose ISDN group (p = 0.49). These patients succumbed to complications of prolonged mechanical ventilation after 2 and 10 days from treatment. Mechanical ventilation was required during the first hour of treatment in 16 patients (80%) in the BiPAP group compared to 4 patients (20%) in the high dose nitrate group (p = 0.0004). Myocardial infarction within 24 h of hospital admission was diagnosed in 11 patients (55%) in the BiPAP group compared to 2 patients (10%) in the high dose ISDN group (p = 0.006). Peak CK was 554 ± 236 IU in the BiPAP group versus 104 ± 95 IU in the high dose nitrates group (p = 0.0001). The combined end point (death, need for mechanical ventilation or MI within 24 h of admission) was observed in 17 patients (85%) in the BiPAP group as compared to 5 patients (25%) in the high dose ISDN group (p = 0.0003).

Secondary end points

The rate of improvement of signs of pulmonary edema was considerably slower in the BiPAP group compared to the high dose ISDN group (Table 3). In the ANOVA analysis, significant time trends were noticed in all three parameters (pulse and respiratory rate and oxygen saturation). In addition, interaction between treatment group and time trend was significant for the three parameters, implying that the change over time in the three treatment groups was significant. Oxygen saturation increased in the BiPAP group from 80 ± 6% at baseline to 89 ± 7% at 50 min compared to an increase from 79 ± 6% to 96 ± 4% in the high dose ISDN group (p = 0.017, Fig. 2). The respiration rate decreased in the BiPAP group from 40 ± 8 breaths/min at baseline to 36 ± 11 breaths/min at 50 min compared to a decrease from 40 ± 5 breaths/min to 31 ± 6 breaths/min in the high dose ISDN group (p = 0.011). Finally, the pulse rate decreased in the BiPAP group from 128 ± 10 beats/min at baseline to 121 ± 18 beats/min at 50 min compared to a decrease from 126 ± 15 beats/min to 104 ± 14 beats/min in the high dose ISDN group (p = 0.014).

Changes in oxygen saturation during study period comparing the high dose IV ISDN Group and the BiPAP group.

Study termination

Due to the significantly high rate of adverse events in the BiPAP-treated group, the study was terminated in the first interim analysis by the safety committee.

Discussion

The results of the present study

The results of the present study indicate that BiPAP ventilation combined with conventional treatment is significantly inferior to high-dose nitrates. This is manifested by increased rate of mechanical ventilation and MI and combined primary end point as well as decreased control of pulmonary edema as demonstrated by slower improvement in pulse and respiration rate and oxygen saturation. As mentioned previously, we have recently compared the use of high dose IV nitrates to conventional treatment in patients with severe pulmonary edema (2). Inclusion and exclusion criteria and baseline characteristics were similar in both studies. In both studies, high dose IV ISDN was administered in the same fashion. However, in the control group of the present study, we have added BiPAP ventilation to conventional treatment of pulmonary edema. A treatment arm with only conventional treatment was not incorporated in the present study due to ethical considerations, since the outcome of patients treated by conventional treatment only was worse in our previous study. In both studies, the outcomes of patients treated by high dose IV ISDN were almost identical. The rate of mechanical ventilation and MI were 20% and 10%, respectively, in the present study as compared to 13% and 17% in the previous study. However, the outcome of the patients treated with BiPAP and conventional treatment in the present study is significantly worse than the outcome of patients treated with conventional treatment only in the previous study. The rate of mechanical ventilation and MI in BiPAP-treated patients in the present study was 80% and 55%, respectively, compared to 40% and 37% in the conventional treatment arm in our previous study.

Therefore, it seems that the addition of BiPAP ventilation to conventional treatment with standard-dose nitrates, furosemide and morphine is detrimental to patients with severe pulmonary edema.

The use of CPAP and BiPAP ventilation in the treatment of pulmonary edema has been reviewed recently (1,3). The results of most previous studies showed a moderate benefit in the use of CPAP regarding improved oxygenation, reduced need for mechanical ventilation and even reduced mortality. Therefore, CPAP was endorsed by many authors for the treatment of pulmonary edema (1,2,4). Ventilation with BiPAP has been examined previously in a few studies (5–7). Most of these studies recruited a small number of patients and the stratification of baseline characteristics was not balanced, making interpretation of the results difficult. However, it seems that BiPAP ventilation, by applying a higher inspiratory pressure and lower expiratory pressure, improves indexes of pulse and respiration rate and oxygen saturation more than CPAP ventilation, without any effect on the rate of mechanical ventilation. Some authors, however, have noticed an increased rate of MI (5,7).

Interpretation of the present study

Although extensively investigated throughout the century, the exact mechanism of pulmonary edema is still largely unknown. In most patients, cardiogenic pulmonary edema is caused by an acute increase of left ventricular end-diastolic pressure (LVEDP) that is transmitted backward to the pulmonary veins inducing fluid exudation to the pulmonary interstitium and alveoli. This increase in LVEDP is usually the result of acute ischemia, which decreases left ventricular diastolic function (thereby increasing LVEDP directly) and systolic function. It has recently been suggested (8) that pulmonary edema is the end result of a vicious cycle in which the decrease in cardiac output is compensated by peripheral vasoconstriction leading to an increase in systemic vascular resistance and afterload. However, if the peripheral vasoconstriction is excessive, the significant increase in afterload results in a further reduction in cardiac output leading to more vasoconstriction and afterload increase. This vicious cycle induces a progressive increase in LVEDP resulting in pulmonary edema. In the present study, high dose IV ISDN administration was more effective than BiPAP ventilation in controlling pulmonary edema. Intravenous nitrates at both standard and high doses induce venodilatation, therefore reducing LVEDP directly. However, when administered at high dose, nitrates induce significant arteriodilatation, therefore, reducing afterload (9) and increasing cardiac output (10). Accordingly, high dose nitrate administration by decreasing afterload may alleviate both the decrease in cardiac output and the increase in LVEDP. Furthermore, this reduction in LVEDP when combined with improved oxygenation (induced by a more rapid improvement of pulmonary congestion) may contribute to faster abortion of ischemia (if present) and prevention of MI.

However, BiPAP and CPAP ventilation improve control of pulmonary edema predominantly by their effect on the lung. These noninvasive ventilation methods improve pulmonary compliance (11,12), reduce atelectasis and intrapulmonary shunting and increase the functional residual capacity. The BiPAP ventilation, particularly, increases tidal volume even more than CPAP and reduces the work of breathing (13). The effects of CPAP and BiPAP on the cardiovascular system are controversial. Both increase intrathoracic pressure, which induces a decrease in preload and afterload. However, the increased intrathoracic pressure per se may reduce stroke volume directly. It is possible that this would lead to an increase in LVEDP, reduce control of pulmonary edema and increase the need for mechanical ventilation. The reduced control of pulmonary edema and elevated LVEDP may increase ischemia and rate of MI.

The results of the present study are in conflict with previous studies. This might be explained by the more severe pulmonary edema in the present cohort.

Baseline oxygen saturation of patients included in the present study was 80% corresponding to Po2 of <50 mm Hg while in most studies demonstrating the efficacy of CPAP and BiPAP ventilation, patients included had much higher baseline Po2. Furthermore, in both the present study and our previous one (2), the treatment of pulmonary edema was administered by mobile intensive care unit teams at the patient’s home or in the ambulance. Accordingly, it is possible that the conditions of some of the patients treated in previous studies would have deteriorated significantly during the initial treatment and transportation, and they would have required intubation and mechanical ventilation prior to arrival to the emergency department. This would introduce a further bias, causing a recruitment drift toward milder cases in the previous studies, explaining the lower baseline oxygen saturation in the present study. Therefore, it is possible that noninvasive ventilation is effective only in patients with mild-to-moderate pulmonary edema. In such patients, the improved oxygenation achieved is probably sufficient to initiate a gradual improvement in the patient’s clinical condition that is later enhanced throughout the gradual build-up of medical therapy.

However, in patients with severe pulmonary edema, the decrease in cardiac output and increase in LVEDP are probably more pronounced at baseline. In such patients, further decrease in stroke volume induced by increased intrathoracic pressure might be detrimental, resulting in patient condition deterioration toward respiratory failure, mechanical ventilation, ischemia and MI.

Therefore, the results of both the present study and our previous one (2) substantiate the notion that the target of treatment in severe pulmonary edema should be decreasing the excessive vasoconstriction and afterload, thereby improving cardiac output. This vasodilatation could be achieved by high dose nitrates and perhaps in the future with enthotelin antagonists. These treatment modalities should be preferred over the nonspecific and possibly harmful attempts to improve oxygenation by noninvasive positive pressure ventilation.

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